In recent years, substantial effort has been made to produce an aluminum alloy sheet which is suitable without modification for the manufacture of both container bodies and container ends. Aluminum beverage containers are generally made in two pieces, one piece forming the container sidewalls and bottom (collectively referred to herein as "container body") and a second piece forming the container top. Using methods well known in the art, a container body is formed by cupping a circular blank of aluminum sheet and then drawing and ironing the cupped sheet by subsequently extending and thinning the sidewalls by passing the cup through a series of dies with diminishing bores. The result is an integral body with sidewalls thinner than the bottom. A common alloy used to produce container bodies is AA 3004 (an alloy registered with the Aluminum Association) whose characteristics are appropriate for the drawing and ironing process due primarily to low magnesium (Mg) and manganese (Mn) concentrations.
However, alloys such as AA 3004 having low magnesium content usually possess insufficient strength to be used for the fabrication of container ends with easy open "ring pulls" or the like. Therefore, alloys with a higher magnesium concentration, such as AA 5082 or AA 5182 alloys, are used for container ends. Table 1 provides a comparison of the major components of alloys AA 3004, 5082 and 5182, as well as other alloys discussed herein.
TABLE 1 __________________________________________________________________________ (weight %) * Alloy Mn Mg Si Cu Fe Ti Cr Zn __________________________________________________________________________ AA 3004 1.0-1.5 0.8-1.3 0.30 0.25 0.70 -- -- 0.25 AA 5082 0.15 4.0-5.0 0.20 0.15 0.35 0.10 0.15 0.25 AA 5182 0.20-0.50 4.0-5.0 0.20 0.15 0.35 0.10 0.10 0.25 U.S. Pat. No. 0.2-0.7 4-5.5 0.3 0.2 0.3 0.1 0.2 -- 3,560,269 AA 5017 0.6-0.8 1.3-2.2 0.15-0.4 0.18-0.28 0.3-0.7 -- -- -- Melt: 0.8 1.5 0.2 0.1 0.4 0.04 -- -- 75% 3004 25% 5182 Adjusted 0.4-1.0 1.3-2.5 0.1-1.0 0.05-0.04 0.1-0.9 0-0.2 -- -- Melt U.S. Pat. No. 0.5-2.0 0.4-2.0 .ltoreq.0.5 .ltoreq.0.5 .ltoreq.1.0 .ltoreq.0.1 .ltoreq.0.2 .ltoreq.0.5 3,787,248 __________________________________________________________________________ * The remainder being aluminum.
A completed container (a body together with an end) must be able to withstand an internal pressure of at least about 60 pounds if it is to contain unpasteurized beer and at least about 90 pounds if it is to contain pasteurized beer, soda pop, or any beverage having similarly high carbonation levels. Currently, containers fabricated from AA 3004 body alloy and AA 5082 end stock are able to withstand 90 pounds of internal pressure if fabricated from aluminum sheet having a gauge of about 0.0116 inches. Containers made from thinner gauges employ less sheet material than those made from thicker gauges and are therefore less expensive to produce. However, containers made from thinner gauge stock, such as 0.0110 inches, have not been sufficiently strong to withstand 90 pounds of internal pressure or have not been sufficiently strong to survive the rigors encountered during long distance transportation.
Another desirable characteristic of an aluminum alloy sheet which is to be drawn and ironed is that the sheet have a low earing percentage. As used herein, the term "earing percentage" (also referred to herein as "earing") refers to the 45.degree. earing or 45.degree. rolling texture. This value is determined by measuring the height of ears which stick up in a drawn cup minus the height of valleys between the ears. This difference is divided by the height of the valleys times 100 to convert to a percentage. The 45.degree. earing is measured at 45.degree. to the longitudinal axis of the strip. Due to this earing, the rim of the shell often becomes deformed and takes on a scalloped appearance.
Because this earing must be removed before the container body is completed, waste occurs. Furthermore, excessive earing, greater than about 2 percent as measured by the Olsen cup test, may also interfere with the drawing apparatus. Minimizing earing helps to minimize waste and simplifies the production process.
One step that has been used to reduce earing is to reduce the cold work percentage (or the percent thickness reduction during the step of cold rolling an alloy sheet). As illustrated in FIG. 1, when AA 5017 alloy is employed, earing decreases as the cold work percentage decreases. However, as further illustrated in FIG. 1, the yield strength also decreases as the cold work percentage decreases. Therefore, increasing the cold work to form stock with thinner gauges or greater strength produces unacceptably high earing. Conversely, reducing the earing by reducing the cold work results in thicker stock with relatively low strength.
Aluminum alloys may be produced by direct chill casting of molten alloy into ingots which are then rolled into strips or may be produced by a continuous strip casting process. Apparatus for continuous strip block casting is described in U.S. Pat. Nos. 3,709,281, 3,744,545, 3,747,666, 3,759,313 and 3,774,670. Although there exist numerous variations of the continuous block casting process, all of the processes generally include the steps described hereinbelow.
Molten aluminum alloy is injected through a nozzle or distributor tip into a cavity formed between two sets of oppositely rotating chilled blocks. While in the cavity, the alloy cools and solidifies to form an aluminum sheet. The aluminum sheet then passes between rollers to further reduce the thickness of the strip. This is typically referred to as hot rolling.
As the continuous strip comes out of the hot rolling step, it is coiled and allowed to cool. The cooled coil is then cold rolled to reduce its thickness still further. Often, the strip will be cold rolled in several passes with an intermediate annealing step between each cold rolling pass.
When the alloy strip has been reduced to its final thickness, it can be cut into appropriate shapes for the production of useful products, such as container bodies or container ends. Typically, at various stages of the process, scrap is produced (plant scrap).
Several patents pertain to low earing aluminum alloys or processes for their production. For example, U.S. Pat. No. 4,238,248 by Gyongyos et al., issued on Dec. 9, 1980, discloses a process for producing a low earing aluminum alloy. A melt of 3004 alloy, or an alloy in which the combined concentration of manganese and magnesium is between 2 percent and 3.3 percent (unless otherwise indicated, all percentages will be weight percent) and in which the ratio of magnesium:manganese is between 1.4:1 and 4.4:1, is cast and then held for 2 to 15 minutes between 400.degree. C. and the alloy's liquidus temperature (the temperature at which the alloy's phase changes between a liquid and a solid/liquid composition, in this case, approximately 600.degree. C.). It is then hot-rolled at a temperature between 300.degree. C. and the non-equilibrium solidus temperature (the temperature at which the alloy's phase changes between the solid/liquid composition and a completely solid composition), coiled and cooled to room temperature. A first cold rolling stage reduces the thickness by at least 50 percent and is followed by a flash annealing stage at 350.degree. C. to 500.degree. C. for less than 90 seconds. A second cold rolling stage results in further reduction of up to 75 percent.
U.S. Patent No. 3,560,269 by Anderson et al., issued on Feb. 2, 1971, discloses an aluminum alloy, the composition of which is set forth in Table 1. An ingot is cast by direct chill casting, heated to 800.degree. F., and held at that temperature for 24 hours. The ingot is hot-rolled and the resulting strip is annealed at 700.degree. F. A first cold rolling stage reduces the thickness by at least 85 percent and is followed by annealing at 600.degree. F. An optional second cold rolling stage provides further reduction of at least 30 percent to a final thickness. The resulting sheet is described as having earing of not more than 3 percent, an amount which, according to the inventors, is acceptable.
As noted above, the required characteristics of alloy for container ends differ from those of container bodies; melting recycled aluminum containers (a combination of ends and bodies) produces a melt which may be unsatisfactory for the production of either container bodies or container ends. The weight percents of the components of a typical melt of recycled aluminum comprising approximately 25 percent container ends and 75 percent container bodies are shown in Table 1. Efforts have been made to produce an alloy from recycled aluminum containers which is suitable for both container bodies and container ends.
U.S. Patent Nos. 4,411,707 by Brennecke et al., issued on Oct. 25, 1983; 4,282,044 by Robertson et al., issued on Aug. 4, 1981; 4,269,632 by Robertson et al. issued on May 26, 1981; 4,260,419 by Robertson et al. issued on Apr. 7, 1981; and 4,235,646 by Neufeld et al. issued on Nov. 25, 1980 disclose related methods for processing recycled aluminum containers. All begin with an initial melt of approximately 25 weight percent container ends and approximately 75 weight percent container bodies, as shown in Table 1. The initial melt is then adjusted, generally by the addition of pure aluminum, to form an alloy whose composition is also shown in Table 1. The combined concentration of manganese and magnesium is within the range of 2.0 to 3.3 percent and the ratio magnesium:manganese is within the range of 1.4:1 to 4.4:1.
The differences among the foregoing patents occur in the way the alloy is cast and processed after being adjusted to the desired composition.
U.S. Patent Nos. 4,235,646, 4,260,419 and 4,282,044 each disclose a continuous strip casting process in which the alloy strip (having the composition previously described) is held at a temperature between 400.degree. C. and 600.degree. C. for 2 to 15 minutes after it has been cast. It is then hot-rolled for a thickness reduction of at least 70 percent, coiled and allowed to cool to room temperature. The strip is then uncoiled and cold rolled to a final thickness in either one or two steps. If cold rolling occurs in two steps, the first results in a reduction of at least 50 percent and is followed by a flash anneal in which the alloy is heated to between 350.degree. C. and 500.degree. C. and then cooled down to room temperature, all within a period not exceeding 90 seconds. The alloy is cold rolled a second time producing additional reduction of 75 percent or less.
U.S Pat. No. 4,269,632 and 4,260,419 disclose direct chill casting methods of the melt described above in which the resulting cast ingot is held at a temperature between 550.degree. C. and 600.degree. C. for 4 to 6 hours and then allowed to cool. It is hot-rolled when its temperature is between 450.degree. C. and 510.degree. C. producing a thickness reduction of between 40 percent and 96 percent. The resulting strip is hot-rolled a second time for an additional reduction of between 70 percent and 96 percent. The strip is coiled and then annealed in one of two ways. It may be flash annealed for 30 to 90 seconds between 350.degree. C. and 500.degree. C. or, it may be annealed for 2 to 4 hours between 315.degree. C. and 400.degree. C. After annealing, the strip is allowed to cool and is then cold rolled in one or more stages to produce a total reduction of approximately 89 percent in thickness. After each cold rolling stage, the alloy is annealed using either a flash or conventional method.
U.S. Pat. No. 4,411,707 discloses a process for producing container ends from the previously described scrap melt using a variation of the continuous chill roll casting method. The molten alloy, between 682.degree. C. and 710.degree. C., is cast to a thickness between 0.23 and 0.28 inches and then rolled to reduce the thickness to approximately 25 percent. The strip is coiled and allowed to cool to room temperature after which it is cold rolled in at least two stages. In the first, a reduction of at least 60 percent in thickness occurs and in the second, a reduction of at least 85 percent occurs. The alloy is annealed for approximately 2 hours at 440.degree. C. to 483.degree. C. between the two cold rolling stages. Additional cold rolling/annealing stages can be used if desired.
U.S. Pat. No. 3,787,248 by Setzer et al., issued on Jan. 22, 1974, also discloses a process for producing an alloy from a melt of recycled aluminum containers which is suitable for both container ends and container bodies. The composition of the alloy is set forth in Table 1. Any conventional casting method may be used (although a preference is stated for direct chill casting) after which the alloy is homogenized for 2 to 24 hours between 850.degree. F. and 1150.degree. F. The metal is then hot-rolled at least twice, the first time achieving at least a 20 percent reduction in thickness at a temperature between 650.degree. F. and 950.degree. F. and the second, also achieving at least a 20 percent reduction, between 400.degree. F. and 800.degree. F. A third rolling operation (comparable to cold rolling), at a temperature less than 400.degree. F., achieves at least a 20% reduction to the final thickness. The alloy is then annealed between 200.degree. F. and 450.degree. F. for a period greater than 5 seconds (preferably between 30 minutes and 8 hours). Instead of a single cold rolling step, the aluminum strip may be cold rolled and annealed two or three times to obtain the final thickness.
U.S. Pat. No. 4,318,755 by Jeffrey et al., issued on Mar. 9, 1982 discloses an aluminum alloy, the composition of which is set forth in Table 1, suitable for container bodies made from recycled containers using continuous strip casting methods. The strip exits the caster at 380.degree. C. to 450.degree. C. and is hot-rolled to reduce the thickness between 72 percent and 82 percent; the strip exits the hot-roller between 150.degree. C. and 200.degree. C. and is coiled. The strip is then cold rolled to its final thickness and is either annealed for 2 hours between 400.degree. C. and 420.degree. C. or flash annealed.
It would be useful to provide a process for the fabrication of an aluminum alloy sheet which has a low earing percentage, which possesses good strength characteristics in thinner gauges than are presently employed and which is suitable for use in the production of both container bodies and container ends. It would also be useful to fabricate such a sheet from an alloy which can be produced substantially from recycled aluminum containers.